Semiconductor processing often involves etching the workpiece, e.g., wafer, substrate, or flat panel, for a specified thickness. The material to be removed may not be the only film on the surface of the substrate. For example, a substrate surface may include both nitride and oxide layers in different areas. A nitride may be silicon nitride, silicon oxynitride, germanium nitride, among others. An oxide may be silicon oxide, hafnium oxide, zirconium oxide, titanium oxide, or germanium oxide, among others. In a feature such as a trench having a bottom and sidewalls, nitride film may line the sidewalls and the bottom. An oxide film may be deposited onto the nitride film. A feature may have a high aspect-ratio, where the opening across the top is small relative to the depth of the feature, or a low aspect-ratio, where the opening across the top is large relative to the depth of the feature. When more than one film is present on the surface of the substrate, etching one film generally involves etching the other; however, etching of the other film may be desirable in some circumstances and not desirable in other circumstances. Controlling relative etch rates of one material to another, i.e., modulating etch selectivities, allows selective etch of one film relative to others in some circumstances and etching of all films on the surface in other circumstances.
In some cases, the top layer of a substrate surface may consist of only one kind of film, but at different thicknesses. For example, an oxide film may line the sidewalls and bottom of a trench as well as cover the top surface outside of the trench. The oxide film may be thicker at the top surface and trench bottom while thinner at the sidewalls. In this case, conformal etching may remove the oxide completely in some areas (e.g., the sidewalls) while leaving some behind in other areas (e.g., the top surface and trench bottom). Where the oxide is completely removed, the underlying film becomes exposed to the etching process and may be etched also. In some circumstances, etching of the underlying film is undesirable. Such undesired etching may be limited by modulating etch selectivities.
“Selectivity” or “etch selectivity” is defined as the ratio of etch rate of one material, the reference material, relative to another material, the material of interest. In a preferred embodiment, the reference material is silicon oxide (SiO2) and the material of interest is silicon nitride (Si3N4). In particular, the silicon nitride may be made by low pressure chemical vapor deposition (LPCVD). In another embodiment, the reference material is thermally-grown silicon oxide (t-SiO2) and the material of interest is another silicon oxide, e.g., high density plasma chemical vapor deposition (HDPCVD) deposited SiO2. A precise way to refer to silicon nitride etch selectivity may be “etch selectivity of SiO2 over Si3N4” or “etch selectivity of SiO2 to Si3N4”, instead of “etch selectivity of Si3N4”. The SiO2 in these phrases is the reference material to which the etch rate of the selected material (i.e., the material of interest) is compared. In the preferred embodiment, the phrases “etch selectivity of Si3N4”, “etch selectivity to Si3N4” or “silicon nitride etch selectivity,” without more, imply that the reference material is thermal silicon oxide. However, the etch selectivity need not always be defined relative to silicon oxide as the reference material. For example, a different reference material may be used explicitly, e.g., etch selectivity of zirconium oxide over LPCVD silicon nitride. An increase in etch selectivity means that the selected material, or material of interest, is harder to etch. A decrease in etch selectivity means that the selected material is easier to etch.
Besides selectivity, the absolute etch rate also may be important. Semiconductor processing involves hundreds of steps and many of them are etch processes. In certain circumstances, it may be desirable to have a very low or a very high etch rate. However, a very low etch rate may not be practical even if technically feasible, because the throughput may be negatively impacted. Thus for each semiconductor processing step, the etch selectivities and absolute etch rates are balanced against each other to maximize throughput while keeping undesired etching to a minimum.